1. Field of the Invention
This invention relates to a recording sheet. More particularly, it relates to a recording sheet utilizing an improved color developer.
2. Description of the Prior Art
Recording sheets in which the coloring reaction of an substantially colorless electron donating organic compound (hereinafter, designated "color former"), for example, such as Malachite Green Lactone, Benzoyl Leuco Methylene Blue, Crystal Violet Lactone, 3-Dialkylamino-7-dialkylaminofluoran and 3-Methyl-2,2'-spirobi(benzo(f)chromene), with an electron accepting adsorptive or reactive compound, which develops color in contact with the color former (hereinafter, designated "color developer"), is used.
As recording sheets in which the above phenomenon is utilized practically, there can be mentioned a pressure sensitive copying paper (for example, see U.S. Pat. Nos. 2,505,470; 2,505,489; 2,550,471; 2,548,366; 2,712,507; 2,730,456; 2,730,457; and 3,418,250 etc.) and a heat sensitive copying paper (for example, see Japanese Patent Publication No. 4160/68, U.S. Pat. No. 2,939,009 etc.). Further, a printing method is known in which an ink containing a color former is applied to a sheet coated with a color developer through a medium such as a stencil to form a colored image (see German Laid-open Specification No. 1,939,962 etc.).
In many cases the coloring reaction of the color former requires pressure from a ball point pen or typewriter, heat or other physical conditions.
One typical embodiment of such a recording sheet is a pressure sensitive copying paper. The pressure sensitive copying paper is obtained by dissolving a color former in a solvent such as chlorinated paraffin, alkyl naphthalene, alkylated diphenylethane alkylated diphenylmethane and alkylated diphenyl, dispersing the solution in a binder or encapsulating it in microcapsules, and coating the dispersion or microcapsules on a support such as paper, plastic film and resin-coated paper.
A heat sensitive copying paper is obtained by coating a color former together with a heat-fusible substance such as acetanilide on a support. In this case, the term "heat-fusible substance" means a substance which is fused on heating and dissolves the color former. A color developer may be coated or impregnated as an ink. In general, a color former and a color developer each is coated on the same surface or opposite surfaces of a support or on different supports respectively.
As color developers, in general acid materials, for example, clays such as Japanese acid clay, activated clay, atapulgite, zeolite and bentonite, organic acids such as succinic acid, tannic acid, gallic acid and phenol compounds, and acid polymers such as phenol resins are suitable.
We, the inventors, have proposed previously that a metal compound of an aromatic carboxylic acid was effective as a color developer for a recording sheet. That is, the performance of a color developer was found to be improved substantially by using a metal compound of an aromatic carboxylic acid. However, the color developing capacity and film surface strength of the coated layer were not satisfactory depending on the preparation of color developer coating solution, thus still leaving room for improvement.